The present invention has utility in providing secure voice communications in a communications system such as, but not limited to, a world-wide satellite cellular communications system. The need for fast, accurate, and cost-effective voice encryption is important in such a system, because many subscribers desire to maintain privacy and confidentiality in their communications.
Communications systems, such as cellular radio telecommunications systems and others, may include system-related components and populations of mobile units. The mobile units may freely move throughout the regions covered by the communications systems. The system components may include base units, repeaters, control stations, switching offices, and the like, and they are controlled by operators and providers of the communications systems. Members of the populations of mobile units may communicate with each other and perhaps with equipment coupled to the public switched telecommunications networks through the system components. The mobile units are typically controlled by customers or end users of the communications systems.
In mobile, digital, wireless communications at least two factors make it difficult to provide privacy. First, because the communications link is wireless, eavesdropping can be conducted by someone other than the desired recipient. Secondly, because the communications transceivers (i.e., subscriber units) are mobile and operate upon digital information, portions of data often become lost or mis-ordered in the transmission process.
Lost or mis-ordered data wreaks havoc with communications channels utilizing standard encryption schemes, especially if significant changes in the order of the data occur, or if much data is lost. Error-laden communications over channels which are susceptible to these problems can be corrected using standard error-correction or data-interleaving techniques, but these cause delays in the transmission. Transmission delays in the order of seconds, or tens of seconds, in personal voice communications are extremely annoying and are generally commercially unacceptable.
FIG. 1 shows a pictorial diagram of an environment within which a radio telecommunications system 10 operates. System 10 includes a constellation 12 of satellites 14 placed in relatively low orbits around the earth. In a preferred embodiment, the configuration of constellation 12 allows at least one of satellites 14 to be within view of each point on the surface of the earth at all times.
Due to their low earth orbits, satellites 14 constantly move relative to the earth. In a preferred embodiment, satellites 14 move in orbits at an altitude in the range of 500-1000 kilometers (km) above the earth. If, for example, satellites 14 are placed in orbits which are around 780 km above the earth, then an overhead satellite 14 travels at a speed of around 25,000 km/hr with respect to a point on the surface of the earth. Electromagnetic signals traveling at or near the speed of light between the surface of the earth and a satellite communications node 14 in such an orbit will require a propagation duration of 2-8 msec or more, depending on the satellite's angle of view. Moreover, electromagnetic signals traveling between the surface of the earth and a satellite 14 in such an orbit may experience a considerable Doppler component of frequency shift, the precise value of which is dependent on a source frequency and the satellite's angle of view.
System 10 additionally includes one or more switching offices (SOs) 16. SOs 16 reside on the surface of the earth and are in data communication with nearby ones of satellites 14 through RF communications links 18. Satellites 14 are also in data communication with one another through data communications links 20. Hence, through constellation 12 of satellites 14, an SO 16 may control communications delivered to any size region of the earth. However, the region controlled by each SO 16 is preferably associated with one or more specific geopolitical jurisdictions, such as one or more countries. SOs 16 couple to public switched telecommunications networks (PSTNs) 22, from which calls directed toward subscribers of system 10 may be received and to which calls placed by subscribers of system 10 may be sent.
System 10 also includes a population, with potentially millions of members, of mobile subscriber units 24. Mobile units 24 are configured to engage in communications with satellites 14 over portions of the electromagnetic spectrum that are allocated by governmental agencies associated with various geopolitical jurisdictions. Mobile units 24 communicate with nearby satellites 14 through communications links 26. System 10 accommodates the movement of mobile units 24 anywhere on or near the surface of the earth.
Any number of subscriber information managers (SIMs) 28 may also be included within system 10. Each SIM 28 may maintain a subscriber database that is relevant to only a discrete portion of the population of mobile units 24. The database may include information describing features associated with mobile units 24, rates to be associated with mobile units 24, current locations for mobile units 24, and the like. Each mobile unit 24 is assigned to one of SIMs 28, and that one SIM 28 is considered the "home" SIM 28 for the mobile unit 24. Each SO 16 may communicate with any SIM 28 through constellation 12, PSTN 22, or another communication path.
In general, system 10 is a network of nodes. Each mobile unit 24, satellite 14, SO 16, and SIM 28 represents a node of system 10. All nodes of system 10 are or may be in data communication with other nodes of system 10 through communications links 18, 20, and/or 26. In addition, all nodes of system 10 are or may be in data communication with other telephonic devices dispersed throughout the world through PSTNs 22. Furthermore, system 10 includes a control station 29 and mobile units 24. Mobile units 24 are controlled by the subscribers of system 10. Control station 29 includes the system components, including satellites 14, SOs 16, and SIMs 28. Control station 29 is controlled and operated by the providers of system 10. When a mobile unit 24 communicates with control station 29, the precise system components involved may be located anywhere in the world, and the communications are routed to the target components through communications links 18, 20, and/or 26. Any one of these or other system components alone or one or more of these or other system components collectively are referred to as control station 29 herein.
Communication services, including calls, may be set up between two mobile units 24 or between any mobile unit 24 and a PSTN phone number. Calls may be set up between any two locations on the earth, assuming appropriate licenses have been obtained in jurisdictions where the locations reside. Generally speaking, each mobile unit 24 engages in system communications with control station 29, and particularly a nearby SO 16, during call setup and during a registration process. The call setup communications take place prior to forming a communication path between a mobile unit 24 and another unit, which may be another mobile unit 24 or a PSTN phone number.
FIG. 2 shows a conceptual block diagram of a prior art voice encryption system in a terrestrial communications system which does not have dynamically allocatable nodes in the network, so which thus has fixed communications links. Information in the form of voice signals, for example, are detected by microphone 31 and processed by appropriate audio circuitry 34. The audio signal is then digitized by vocoder 37, and the digital signal is fed into modulo-two summer (an exclusive-OR combinational logic circuit) 50. A first crypto-algorithm generator 49 generates a unique binary sequence that is fed over line 40 to modulo-two summer 50, where it is mixed with the digital signal from vocoder 37 and output to transmitter or transceiver 51.
Transmitter 51 transmits the encrypted digital signal to receiver or transceiver 55 via any appropriate communications link 52, such as a radio-frequency link, cable, etc. The received encrypted digital signal is fed into modulo-two summer 74, which also receives the identical binary sequence from a second crypto-algorithm generator 69. Modulo-two summer 74 decodes the original digital signal, which is converted to an analog signal by digital/analog converter 77, processed by audio circuitry 80 and fed into speaker 83.
In order for modulo-two summer 74 to properly decode the original digital signal from the received encrypted digital signal, crypto-algorithm generator 69 must be synchronized with crypto-algorithm generator 49. Such synchronization is typically achieved via a separate synchronization link 58 coupling the two crypto-algorithm generators. However, it will be understood that rather than using two separate channels, one for the encrypted voice and one for the encrypted sync, both could be implemented on the same channel.
For the sake of simplicity of description, in FIG. 2 information is shown being transmitted from Tx 51 to Rx 55, but it should be understood that suitable additional circuitry would normally be provided in order for information to be transmitted in the opposite direction as well.
It will be appreciated that the signal path lengths of the communications link 52 and the synchronization link 58 of the prior art system shown in FIG. 2 are fixed. Thus the system depicted in FIG. 2 has a significant disadvantage when used in a communications system in which the signal paths may be changing.
For example, in a world-wide cellular communications system utilizing low-earth orbit (LEO) satellites, synchronization cannot be readily maintained between the crypto-algorithm generators due to the dynamic aspects of the communications links. This is because the link distances within the system are constantly changing -e.g., the distance between an individual subscriber unit (ISU) and a LEO satellite, the distance between a LEO satellite and a gateway, the distance between cross-link LEO satellites, etc.
Similar problems may be encountered with a terrestrial system in which the link distances within the system are changing -e.g., in a digital switched network having dynamic allocation of nodes.
Thus there is a significant need to provide voice and/or data encryption in a communications system having noisy communications channels that lose and mis-order bits, and wherein such encryption does not cause any significant transmission delay.
There is also a significant need within a communications system having dynamic signal path lengths, such as a world-wide cellular communications system utilizing LEO satellites, to provide an apparatus and method for fast, accurate, and cost-effective synchronization of the transmitting and receiving crypto-algorithm generators.